Method and system for controlling strip thickness in a tandem reduction mill



Oct. 6, 1970 L. w. DUNN METHOD AND SYSTEM FOR CONTROLLING STRIP THIcKNEss 1N A TANDEM REDUCTION MILL.

2 Sheets-Sheet 1 Filed March 13, 1968 Oct. 6, 1970 l.. w. DUNN METHOD AND SYSTEM FOR CONTROLLING STRIP THIcKNEss 1N A TANDEM REDUCTION MILL.

2 Sheets-Sheet 2 Filed March 13, 1968 ENTRY EECALIBRATION' PROFILE MEMORY ENTRY TENSION AGC FIG. 2.

INVENTOR Lawrence W. Dunn ATTORNEY WITNESSESZ United States Patent O 3,531,961 METHOD AND SYSTEM FOR CONTROLLING STRIP THICKNESS IN A TANDEM REDUC- TION MILL Lawrence W. Dunn, Williamsville, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 13, 1968, Ser. No. 712,766 Int. Cl. B211) 37/12 U.S. Cl. 72-8 13 Claims ABSTRACT OF THE DISCLOSURE An automatic gauge control system and method is provided for use in a cold rolling tandem strip mill. Ongauge strip is maintained by the individual and cooperative action of an entry or coarse correction system featuring feed forward monitoring, an interstand tension control system, a delivery control system and a strip head end/ tail end compensation system.

BACKGROUND OF THE INVENTION This invention relates, in general, to a method and system for reducing the thickness of a material in a tandem rolling mill, and more particularly, to an adaptive automatic gauge control system used in a tandem cold rolling mill.

Cold mills are used to roll finished at products such as sheet and strip from hot rolled coils of steel or nonferrous metals. Cold rolling is necessary to reduce the material thickness to a desired value when the hot strip mill cannot adequately reduce the material to the gauge quality; or gauge tolerance required. As the demand has increased for lighter gauges and as users have demanded better quality uniformity and gauge tolerances, the cold mill has seen an increased importance. In addition, other advantages were achieved in vcold rolling such as a smoother and denser surface and the impartation of certain mechanical properties.

In a tandem metal rolling mill, a number of rolling stands are placed in a line with the output of one stand serving as the input of the next and with each succeeding stand further reducing the strip thickness. It is well known under principles of mass flow and assuming a constant width strip that, if the thickness out of any stand is constant and if the speeds are maintained in the same relationship, then the thickness out of all of the other stands is held constant. Stated mathematically:

where s1 is the speed for stand 1 and t1 is the thickness at stand 1, etc. In an effort to maintain the equality of the above relationship, various mill schemes have been attempted and have not been altogether satisfactory. For example, the use of a roll force gauge control system similar to that used in a hot rolling mill has not provided the magnitude of accuracy that had been achieved with the heavier gauges usually found in hot mills. Even positioning of an X-ray gauge following one or more stands has been on only moderate value inasmuch as this amounts essentially to an adjustment after the fact; moreover, this adjustment may not be valid for the strip then in the preceding rolling stand. Indeed, the economics alone of placing an X-ray gauge following each stand even if desirable has been prohibitive.

SUMMARY OF THE INVENTION It is therefore a general object of the present invention to provide an improved method and system for controlling strip thickness by adapting succeeding stands in a tandem rolling mill to correct for recognized errors.

3,531,961 Patented Oct. 6, 1970 "ice It is another object of the present invention to provide an improved method and system for controlling strip thickness by the integrating of subsystems which both eliminate errors in the incoming strip and errors generated within the mill itself.

A further object of the present invention is to provide an improved method and system for controlling strip thickness by a process control computer which stores the strip error prole and initiates correction in a succeeding stand or stands.

Gauge deviations in a strip may be grouped generally into two categories-those inherent in the incoming strip, and those generated within the mill itself. In regard to the former, the hot rolled strip which is entering a cold reduction -mill is known to have inherent gauge deviations -which are a result of prior processing (usually in the hot strip mill) and which can be generally described as a function of the conditions that create them. The foremost common types of incoming product errors are as follows:

1) Slowly changing, ramp type deviations occurring as a result of hot mill temperature rundown. The magnitude of these variations from the head end to the tail end of an incoming strip is generally in the range of 2% to 10% of average gauge depending upon whether the hot mill control system was of the regulated or non-regulated type.

(2) Cyclic variations resulting from reheat furnace skid marks. The frequency of these variations is relatively low and is a function of the furnace characteristics and the hot mill drafting practice; their magnitude is of the order of 3% to 5% of average gauge.

(3) High frequency, cyclic variations due to roll eccentricities. The frequency of this type of error is a function of the roll circumference and the r.p.rn.; their magnitude is a function of the degree of roll eccentricity permitted in the mill.

(4) Step function Variations resulting from operator adjustments to mill settings, Weld points butting two hot band coils together, etc. The magnitude and frequency of these variations can be quite random; generally the magnitudes are relatively high.

Although the above classes of deviations were separately mentioned, these variations naturally exist coincidentally.

It is within the principles of the present invention to provide an entry automatic gauge control system to eliminate or substantially reduce these errors. As such, a roll force automatic gauge control system has proven to have the most pronounced effect on correcting the errors described in paragraphs (l) and (2) above. However, it is less capable of correcting the type of errors described in paragraphs (3) and (4) due to the inherent time response of the system; i.e., by the time the system has recognized the error as the result of roll force change, the deviation or error has passed the point at which it could be corrected. It is, moreover, intentional that these high frequency and step change deviations are purposely ignored or filtered from the system.

A second feature of the present invention isa feed forward interstand tension automatic gauge ycontrol system which acts as a follow-up correction for thepreviously mentioned roll force automatic gauge control system for the type of errors described in paragraphs (l) and (2) as well as a primary system for the elimination of the errors described in paragraph (3) and to a large measure those errors described in paragraph (4). Due to its inherent operating nature, this system recognizes the error before its correction is necessory and then makes the correction in phase with its occurrence at the roll bite of the next stand.

In addition to the incoming workpiece variations just described which are essentially strip thickness disturbances, other conditions exist which affect the control of gauge. More generally, these are variations in the metallurgical properties or the strip hardness over the length of the strip. These types of errors may be numerous and their occurrences unpredictable, and their effects usually result in strip thickness deviations resulting from an increased or decreased mill stretch or an erroneous X-ray gauge reading. Extreme differences in steel recrystallization could result in varying outputs from the X-ray gauge even though strip thickness does not deviate. Moreover, variations in composition of the steel cause erroneous thickness readings from the gauge and may also create hard spots throughout the coil. Those types of deviations which result from strip hardness changes can be adequately controlled by a recalibration of the aforementioned roll force automatic gauge control system. In accordance with the principles of the present invention, this recalibration is accomplished by having a process computer periodically compare the expected effect of a screwdown correction to a roll force change with the actual effect measured by an X-ray gauge following the first stand in the rolling mill. Then, if the actual gauge differs from the expected gauge, the system gain is accordingly changed to account Ifor this difference.

Other than errors present in the incoming product, it is well known that there are numerous factors associated directly with the mill itself that contribute to and magnify gauge errors. During the mill accelerating cycle, gauge errors occur which vary as a function of the mill speed. These errors have been attributed mainly to back-up roll bearing oil film thickness changes although other factors are involved. As the mill speed increases, the bearing oil film builds up causing an assoicated reduction in strip thickness in a non-linear fashion. The reverse phenomenon occurs during mill deceleration. Those gauge errors introduced in this fashion tend to disturb the interstand tensions as well. A further feature of the present invention is to employ an interstand tension control by screwdowns for the purpose of nullifying these effects. Operating in combination with the mill speed regulating system, the interstand tension control subsystem has the ability and capacity to reduce the gauge errors created by mill accelerations and decelerations to a level which a vernier automatic gauge control can effectively control.

For any type of mill, mechanical stiffness is an important factor in establishing strip gauge. Theoretically, given an infinitely stiff mill with correspondingly inflexible roll systems, the maintenance of a set strip thickness is almost inherent. However, in practice this is not always possible, and as a result, variations in the incoming strip metallurgy and gauge create forces which deform the mill stand in various ways wherein these deformations exhibit themselves in the form of delivered strip gauge deviations. Closely associated with the mill stiffness concept are roll deformations-flattening, bending and heating. These deformations occur as a result of heavy roll separating forces brought about by heavy percent drafting, steel hardness and other factors. They, too, create gauge variations as well as shape problems which complicate the control of gauge thickness. It is well known that roll bite friction affects strip thickness, and it in turn is altered by roll coolant, strip lubricant (both quantity and mixture),'roll temperature, and roll surface conditions. To reduce these types of mill generated gauge deviations to zero and in cooperation with the forementioned interstand tension control by screwdowns, a delivery automatic gauge control system has been provided. Being a Vernier type control it is adequately suited to handle the relatively low magnitude and slowly occurring errors.

`Other objects and advantages of the present invention will become more apparent in view of the following description when taken in conjunction with the accompanying drawings.

4 DESCRIPTION or THE DRAWINGS FIG. lis a schematic representation of a five stand rolling mill arranged in accordance with the principles of the present invention;

FIG. 2 is a simplified schematic diagram of the rst three stands of the rolling mill of FIG. l wherein the stand 1 is dummied.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. l, a five stand tandem cold rolling mill is shown for successively reducing a strip 8 to a desired final gauge. Each of the stands 1-5 includes a pair of work rolls 1GI and 12 driven by a motor 14. Motor speed is detected by a signal from a tachometer 32 connected to each drive motor. The speed of the drive motor is controlled through a speed controller which, for ease of illustration, has been shown only for stands 1, 4, and 5 by respective symbols S1, S4, and SS. Reduction on the strip 8 as it passes through work rolls 10 and 12 is achieved by applying a predetermined pressure through back rolls 16 and 18 by a screwdown 17. The regulation of the applied pressure is through a screwdown motor 20` whose operation is controlled by a screwdown controller 22. A screwdown position detector 24 monitors the position of the screwdown 17 by detecting the revolutions of the screwdown motor 20. The finished workpiece is then coiled on a reel 26 controlled by a drive motor M.

Between rolling stands and between the last rolling stand 5 and the takeup reel 26 are tensiometers 28 which indicate the interstand tension. X-ray devices for measuring actual gauge are positioned between stands 1 and 2, stands 2 and 3, and stand 5 and the takeup reel 26, and are designated respectively by the symbols X1, X2 and X5. Associated with each rolling stand is a load Cell 30 which measures the roll force at the respective stand.

Control of the rolling process is provided by a process control computer 40 which provides communication between the rolling mill inputs and outputs in a predetermined manner. The exact mode of control is provided by an externally provided program which functionally relates an input or combination of inputs to provide controlled output signals which are commensurate with an on-gauge strip. The functional relationship between and among certain inputs as seen to exist within the process computer will be discussed in detail herein and are generally designated by the blocks within the process control computer 40.

The inventive combination to be further described is comprised of the combination of four subsystems which act together to provide an 'on-gauge finished workpiece. The first subsystem is known as the entry or coarse correction AGC. It is the duty of this entry correction subsystem to remove the incoming strip gauge errors to thereby provide a constant strip gauge for reduction in the later mill stands. This entry or coarse correction AGC is itself comprised of several components the first of which is designated as the stand No. 1 roll force AGC and designated by block 100. The stand 1 roll force load cell 30 provides a gauge deviation signal with the process control computer 40 which relates a roll force change to a change in strip gauge. This gauge deviation signal input along with a correction signal is then translated by the operation of the process control computer 40 in block 100 into an output correction signal to the stand 1 screwdown controller 22 which causes the screwdown 17 to change position at a rate, in a direction, and by an amount calculated to reduce this error to Zero. A feedback signal is received from the screwdown position detector 24 at block 100 for monitoring the change in screwdown. The correction which occurs is as immediate as the system time response permits.

A second subsystem of the entry or coarse correction AGC is an interstand tension AGC by X-ray gauge feedforward monitoring. The X-ray gauge X1 following stand 1 provides the process control computer 40 with a strip gauge error signal as shown in block 102 which the process control computer 40 stores in its memory section as an error profile. The gauge error is mainly that which the stand 1 roll force AGC cannot eliminate or has been unsuccessful in eliminating. The process control computer 40 also based on the measurement of stand 1 speed from the tachometer 32 can appreciate when a given error point in the strip is about to reach the stand No. 2 roll bite and then can initiate control of the stand 1 speed through speed controller S1 in a direction and by an amount necessary to create an interstand tension change which would then cause the strip error to be reduced to zero. The advantage of this type of system is that by virtue of the fact that it is a feedforward system, the correction of the error is anticipated and the correction is made in phase with the occurrence of an error at the correcting point. In particular, take a point on the strip 8 and assume that the X-ray gauge X1 is located as close as possible to stand 1. As the deviation in the workpiece strip passes under the X-ray gauge X1, the gauge will emit a deviation signal which is brought into the computer as shown in block 104 and stored in the computer memory as shown in yblock 102. Now the process control computer 40 knows that at a particular point in time, such an error existed and the computer by virtue of the speed signal representing the speed of the strip between the stands from tachometer 32 of stand 1 will actually track the error in time until the point of error is about to reach the stand 2 roll bite. At the time when the error is at the stand 2 roll bite, the process control computer having delayed a correction until this time, then causes the stand 1 speed to be changed in the direction necessary to remove this error; theoretically, from a corrective standpoint, the correction to the error can best be handled when that error is in the roll bite of stand 2. A serious fault with previous systems using an X-ray gauge monitor has been that the gauge deviation has been detected after the fact and a correction is made at the preceding stand. Thus, Whatever portion of the strip which has traversed from the stand to the X-ray gauge would remain in error. The amount of correction made in this case would be just sufficient to cause an overtension or undertension depending upon what the correction must be between these two stands. In practice this would represent a very small percent change in the running speed of the mill.

It is intended that the output of the X-ray gauge X1 would f be sampled at a very high frequency so that, in effect, an accurate profile of strip deviation over the entire length of the strip ca n be accomplished.

A third subsystem of the entry or coarse correction AGC is the entry system recalibration by X-ray gauge feedback monitoring. The degree of correction necessary to erase an error by either the stand No. 1 roll force AGC or the interstand tension AGC by X-ray gauge feedforward monitoring is dependent on strip metallurgical properties and mainly hardness of the strip. Of course, mill property changes such as roll heating, deformations, roll coolant changes, etc. also create variations in corrective requirements. Therefore, it is incumbent upon the process control computer 40 to adapt itself to instantaneous rolling conditions by monitoring effects of corrections it commands and by altering the degree of corrections, if necessary, to cause an on-gauge end product. This is accomplished in two parts wherein the first consists of the provision of a second input to the roll force AGC in block 100 wherein periodically the process control computer 40 monitors the strip gauge error signal from the X-ray gauge X1 following a command for a screwdown position change. It can be seen in FIG. 1 that the process control computer 40 knows from the X-ray gauge signal X1 the change the screwdowns are required to make by getting a feedback of the actual gauge after the correction has been made; if the correction which is established does not give the expected result it alters the degree of cor- 6 rection made by the screwdowns. The signal from the X-ray gauge X1 is fed into the signal mixer in block 104 which along with other inputsignals acts to provide a recalibration signal to the roll force AGC portion in block for providing a change in the screwdown position as previously described.

The second part consists of providing a correction signal to the interstand tension in block 102 AGC wherein the deviation signal from the X-ray gauge X2 located behind stand 2 is periodically monitored by the process control computer 40 as shown in the mixer block 108. If after a corrective tension change, the expected error reduction is not accomplished, the process control computer 40 then automatically increases or decreases the degree of correction necessary to rezero the system. This is shown by having the output from the mixer block 1-08 connected to block 102 which then alters the speed signal to speed controller S1 to provide a proper zero recalibration. Initially, the calibration of the previously described subsystems are based on a manual input of steel grade and hardness and/or the history of coils of the same order rolled before the one now in process.

The second subsystem employed in the total automatic gauge control system is an interstand tension control by screwdowns. T'he interstand tensions in the mill are regulated by controlling the screw positions of succeeding stands. When the tension goes beyond a pre-established set point dead-band as detected by the process control computer 40 from the respective signals from the interstand tensiometers 28 into the respective tension controllers 106, the process control computer causes the screwdowns on the succeeding stand to position by an amount and in a direction which will return the tension to its pre-established target value.

It has been shown that in a speed regulated mill, interstand tension control regulates for a constant percent reduction of the strip in a given stand and thus the gauge output from the mill follows the gauge profile established at the input end of the mill. Thus, if the entry or coarse correction AGC will remove the incoming gauge errors, the tension control systems will then maintain an on-gauge strip throughout the mill and at the same time prevent or at least minimize deviations being generated in the rolling process itself. This is especially true during times of mill speed changes when the roll opening variations resulting from mill stand characteristic alteration such as oil film change occur.

With such a tension control system, programming or recalibration of system operation during mill acceleration or deceleration periods will become unnecessary. The system will automatically compensate for roll opening variations such as for example, during an acceleration when rolls close as a function of mill speed increase, back tension on the stand decreases. The process control computer 40 then recognizing this tension decrease, will cause the stand screws to open to overcome the tendency for under-gauge strip which would otherwise have been delivered from this stand. It is intended that the process control computer based on a known corrective action (screwdown positioning) and measurement of its result (tensiometer signal), adapts itself to provide the necessary degree of correction to maintain tension at a predetermined amount. The tension control system will cooperate with the entry or coarse correction AGC and the delivery AGC tension subsystems such that when the entry tension AGC calls for an increasing of interstand tensioning between stands 1 and 2 to correct for an over-gauge strip condition and the tension required is excessive, the tension control will then operate to increase the roll pressure on the stand 2 primarly to reduce the tension between stands 1 and 2 and at the same time making the strip reduction which created the increase tension by the entry or coarse correction AGC system initially.

The third subsystem used in this automatic gauge control system is comprised of a delivery (Vernier) AGC system. This vernier correction subsystem will remove the gauge errors which the previously described subsystems have been unable to successfully eliminate and, within limits, will also correct for improper mill setup or gauge deviations generated within the mill. This delivery AGC system is comprised of two parts the first of which is an interstand tension AGC by X-ray gauge feedback monitoring. A third X-ray gauge X is located on the delivery side of stand 5 which measures the delivered strip gauge and provides an output signal proportional to strip error into the delivery tension block 110. The process control computer 40 then upon receiving this signal causes stands 4 and 5 and to a lesser but proportional degree stands 3 and 4 tensions to be changed through adjustment of the stand 5 and stand 4 speeds respectively at a rate by an amount and in a direction to reduce this error to zero. This is accomplished by providing a correction signal to speed controllers S4 and S5 from the delivery tension AGC block 110.

The second part of the delivery AGC system is used to detect trends and operates by a long term integration of the delivery gauge error trend. The process control computer 40, monitoring the delivery X-ray gauge deviation signal from the X-ray gauge X5 and integrates this deviation in block 112 to recognize any trend toward an underor over-gauge condition over a predefined period of time and causes the entry AGC set point to be adjusted through mixer block 104 to reverse the trend by altering the degree of gauge correction made by the entry AGC subsystem. This integrating type of subsystem will nullify and/or correct poor mill setup within limits and likewise insure that the delivery AGC subsystem has suicient range of correction for immediate gauge errors. A correction of any magnitude made by the integrating system is spread across the mill as a result of the operation of the interstand tension control subsystem. For example, if the integrating subsystem detects an over-gauge tendency, it alters the entry AGC system target to produce a thinner gauge output. This causes a higher per unit interstand tension which in turn causes each succeeding stand screwdown to operate to increase the roll force and reduce gauge further. The net result is that increased mill loading because of the over-gauge trend is generally distributed in an even manner in each stand.

The fourth and last subsystem involved in the total AGC system is the strip head end/tail end compensation system. It is known that lack of strip tension during the mill threading process will cause the head end of the strip to be over-gauge as a function of the mill stiffness. As the strip tail end leaves each stand, strip tension between that and the following stand is lost which also results in an overgauge product. To offset these effects as much as possible, the following provisions are made in the system. For proper head end compensation, irnmedately after the entry of the strip into each stand, that stand screw position would be programmed and set to a roll opening of a value equal to that required to establish desired gauge output without the interstand tension. As soon as the strip is entered into stand 2 as determined by a load cell signal from the load cell 30, and interstand tension is established, the stand 1 roll force AGC system is activated. Then while the strip is traversing from stand 1 to stand 2, gauge output from the stand 1 would then be on target. When the roll force AGC system cornes into operation, the roll opening will automatically reset as a function of the subsystem regulation to that value necessary to put out target gauge with the correct interstand tensions established. As the strip progresses from stand to stand, the interstand tension control subsystems are sequentially activated and automatically adjust screw positions to create and maintain interstand tensions at their pre-established values. Simultaneous with the entering of stand No. 3 the entry tension AGC system is activated, and full entry gauge control is then in operation. The cycle is completed when the stand 5 is entered and the delivery AGC is then made operative.

During both acceleration and deceleration of the mill, the entry roll force AGC system is made inoperative because it would function in a direction which would oppose good gauge control. To provide tail end compensation the following sequence of operations would occur: (l) entry tension AGC system is deactivated, (2) the stand 1 and 2 tension control system is deactivated, (3) the stand 2-3 tension control system is deactivated, (4) the stand 2-3 interstand tension is increased a programmed and predetermined amount by reduction of the stand 2 speed. This last step is provided in order to overcome the loss of interstand tension between stands 1 and 2 and to maintain a target gauge once the workpiece is out of the stand 2. As the strip then leaves each succeeding stand the above steps (2), (3) and (4) are repeated for the proper stand involved. Then when the strip 8 leaves the stand 4, the delivery tension AGC system is deactivated.

For certain types of operations, it may be necessary that the stand 1 be dummied out. In this instance as seen in FIG. 2 the entry AGC subsystem is automatically revised to provide a coarse gauge control system without the stand 1 being involved. The method of control that would be used is still the feedforward approach previously described except that instead of the primary control being by the interstand tension adjustments, stand 2 screwdowns are positioned at a rate, by an amount, and in a direction to reduce the gauge error to zero. The strip error profile as measured by the X-ray gauge X1 following stand 1 is still stored in the process control computer memory and is held there until the erroneous strip then approaches stand 2. Once the strip reaches stand 2 the stand 2 screwdowns are operated to remove the anticipated off gauge strip as it enters the stand 2 roll bite. The X-ray gauge X2 following stand 2 is then used to supply a gauge output signal Ato the process control computer 40 in mixer block 104 which is used to check the results of the corrective action taken and to then recalibrate the system if necessary.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of components may be resorted to without departing from the scope and spirit of the present invention such as use in the hot reduction of steel or hot and cold reduction of non-ferrous metals.

What is claimed is.

1. In a control system for a tandem rolling mill having a plurality of rolling stands operative to successively reduce the thickness of a workpiece, a gauge control system comprising the combination of:

means for detecting the roll force of at least one of said rolling stands and for providing a roll force output signal representing the roll force detected for at least one of said rolling stands.

means for detecting the unloaded roll opening of at least one of said roll stands and providing a position output signal corresponding to the unloaded roll opening of at least one of said stands,

means for generating a workpiece thickness signal following a first and last of said rolling stands,

means for controlling the unload roll opening of at least one of said rolling stands,

means for determining and effecting a change in unloaded roll opening of the rst rolling stand in response to the rst rolling stand roll force output signal, said determining means also including means responsive to the last rolling stand workpiece thickness signal for detecting over and under-gauge conditions over a predetermined time period and providing a correction signal representative thereof to said first stand roll force output signal.

2. The tandem rolling mill control system as set forth in claim 1, said control system also including:

means for detecting interstand strip tensions and providing representative signals thereof,

means for controlling the speeds of operation of said rolling stands in response to respective speed control signals,

means for recording a profile memory of said workpiece in response to said first rolling stand thickness signal, means for effecting a gauge correction to said workpiece at a second rolling stand by varying the speed control signal for said first stand to create a predetermined level of interstand tension between the first and second of said rolling stands. 3. The tandem mill control system as set forth in claim 2 wherein said control system further includes means for detecting the speeds of operation of said rolling stands, and wherein said effecting means determines in reponse to at least said speed signals and said profile memory when a gauge error is at said second stand to thereby alter the said first stand speed control signal.

4. The tandem mill control system as set forth in claim 2 wherein said control system also includes:

means for generating an intermediate workpiece thickness signal in a stand following said first stand,

means responsive to said intermediate workpiece thickness signal for providing a speed recalibration signal to said first stand speed controlling means.

5. The tandem mill control system as set forth in claim 4, wherein said control system includes means for effecting a predetermined level of interstand tension between predetermined rolling stands in response to Said last stand workpiece thickness signal.

6. The tandem mill control system as set forth in claim 5, `wherein said determining and recording means include a digital computer system, said computer system having inputs coupled to said detecting and generating means, and outputs coupled to said speed and unloaded roll spinning controlling means, and a programming system for said computer system operative to make the speed and unloaded roll opening determinations.

7. The tandem mill control system as set forth in claim 6, wherein said programming system includes a coarse correction control program which predictively determines a corrective unloaded roll opening at said first stand, and a first stand speed level in accordance with a second stand speed level to develop a predetermined interstand tension between said first and second stands at a level necessary to substantially remove previously uneliminated incoming gauge errors.

8. The tandem mill control system as set forth in claim 7, wherein said programming system includes an entry calibration control program which recalibrates said first stand speed and unloaded roll opening in accordance with the degree of correction required in said coarse correction control program.

9. The tandem mill control system as set forth in claim 8, wherein said entry recalibration system also includes recalibration of said first stand speed and unloaded roll opening in accordance with a detected trend of deviations of the last stand exit gauge of the workpiece.

10. The tandem mill control system as set forth in claim 6, wherein said programming system includes a Vernier correction program system for determining speed of at least the two last stands to maintain an interstand tension level sufficient to remove previously uneliminated gauge errors.

11. The tandem mill control system as set forth in claim 2, wherein said first stand effects no reduction in the workpiece gauge.

12. A method for controlling a tandem Arolling mill having a plurality of rolling stands operative to successively reduce the thickness of a workpiece wherein said method comprises the steps of detecting the roll force of at least one of said rolling stands and providing respective roll force output signals representing the roll forces detected, detecting the unloaded roll opening of at least said one of said rolling stands and providing respective position output signals corresponding to the respective unloaded roll openings of said stands, u

generating a workpiece thickness signal following a first and last of said rolling stands,

controlling the unloaded roll openings of the latter said stands,

determining and effecting a change in unloaded roll opening in said first rolling stand in response to said first stand roll force output signal, and detecting the said last stand workpiece thickness over a predetermined time period to provide a correction signal representative thereof to the said first stand roll force output signal.

13. The method for controlling a tandem mill as set forth in claim 13 wherein said method also includes:

determining the interstand strip tensions and providing representative signals thereof,

controlling the speed of operation of said rolling stands in response to respective speed control signals, recording a profile memory of said workpiece in response to the first rolling stand thickness signal, and effecting a gauge correction to said workpiece at the second rolling stand by varying the speed control signal for said first stand to create a predetermined level of interstand tension between the latter stands.

References Cited UNITED STATES PATENTS 3,232,084 2/1966 Sims 72-16 3,328,987 7/1967 Feraci 72-8 3,355,918 12/1967 Wallace 72--16 3,357,217 12/ 1967 Wallace et al. 72-8 3,416,339 12/1968 List 72-8 3,448,600 6/ 1969 Coleman et al. 72-8 MILTON S. MEHR, Primary Examiner 

